Crystals of KTiOPO.sub.4 and its analogs are considered highly useful because of their nonlinear optical properties. U.S. Pat. No. 3,949,323 teaches the use of flaw-free crystals r optical devices. Recently higher AC conductivity has been correlated with increased damage to crystals. See Morris et al., "Defects in KTiOPO.sub.4 ", Proc. Book MRS Optical Matls., 1989. Relatively large crystals with low ionic conductivity are thus considered particularly desirable for many optical applications. Since KTiOPO.sub.4 and its analogs are known to decompose upon melting, hydrothermal and flux methods have commonly been used to grow crystals of these compounds. U.S. Pat. No. 3,949,323, as well as others teach preparation of the crystals by hydrothermal methods. Indeed, despite the requirement for high pressures (on the order of hundreds of atmospheres), high temperatures, and relatively long crystal growth times, the art has generally conveyed a preference for hydrothermal methods of crystal growth. See R. A. Laudise et al., J. Crystal Growth 74, 275-280 (1986). R. F. Belt et al, Laser Focus Electro-Optics, 110-124 (October 1983) specifically advises methods other than hydrothermal.
A desire for larger crystal size, better quality and greater durability, as well as the disadvantages of hydrothermal processes have led to continued interest in flux growth techniques and to the development of a variety of flux processes. In U.S. Pat. No. 4,231,838 crystal growth is carried out by heating certain mixtures of MTiOXO.sub.4 with a nonaqueous flux M/X/O (where M is selected from K, Tl, and Rb and X is selected from P and As) or their precursors to produce a nonaqueous melt. Crystal growth is affected by the use of a temperature gradient or by slow cooling of the melt at a rate of not greater than 5.degree. C./hour. This flux process reportedly provides an economical, low pressure process which may be used as an advantageous alternate to hydrothermal processes. A substantially isothermal process for producing flaw-free single crystals of MTiOXO.sub.4, wherein M and X are defined as above, is described in U.S. Pat. No. 4,761,202. This method involves heating certain melts of MTiOXO.sub.4 and a flux of M and X oxides to the seeding temperature, suspending a seed crystal of MTiOXO.sub.4 in the melt, slowly decreasing the temperature of the melt while maintaining substantially isothermal conditions, and continuing the temperature decrease until the desired crystallization of MTiOXO.sub.4 on the seed is completed. This method produces relatively large crystals. The ionic conductivities of the crystals resulting from flux processes can be undesirably high for many optical applications (e.g., greater than 10.sup.-6 ohm-cm.sup.-1,).
Some flux growth methods have included the use of other fluxes to improve various aspects of crystal production. The use of tungstic anhydride is described by Ballman, et al., "Growth of Potassium Titanyl Phosphate (KTP) from Molten Tungstate Melts" J. of Crystal Growth 75, 390-394 (1986) to improve the yield of quality crystals. Japanese Patent 63 40,799 discloses that the use of certain tungsten, molybdenum, chromium and sulfur based fluxes avoids prolonged retention time and suppresses precipitation of black plate-shaped crystals. There remains a need for methods to conveniently reduce the ionic conductivity of KTiOPO.sub.4 -based optical crystals.